1822.] Smelting of Tin Ores in Cornwall and Devonshire, 449 



parts of arsenic. Notwithstanding that the excess of oxide of 

 iron increases the volume of the precipitate, it contributes to 

 render the subarseniate less gelatinous, and more easy to edulco- 

 rate. The subarseniate ought to be twice heated, in order to be^ 

 certain that it ceases to lose weight ; for a small quantity of sul-* , 

 phuric acid often adheres very strongly to this precipitate. 



As the arseniate of barytes is also an insoluble compound, I 

 endeavoured to separate the arsenic acid from a neutral solution 

 by muriate of barytes ; but this method is attended with such 

 inconveniences that it cannot be employed. In the first place, 

 the arseniate of barytes which is precipitated is usually a mixture 

 of neutral and subarseniate, and the liquid becomes acid, as hap- 

 pens with the phosphates of barytes, lime, &c. When then 1 

 wished to precipitate subarseniate of barytes, by adding excess 

 of ammonia, I found that a considerable portion of subarseniate 

 remained dissolved in the excess of alkah, as it is well known to 

 happen with arseniate of lime. Lastly, I found that when arse- 

 niate of barytes is washed, the water never ceases to give a prcr-.^ 

 cipitate with sulphuric acid. I relate these circumstances ia' 

 order to save others from fruitless labours. 



Article XIII. 



'O 



On the Smelting of Tin Ores in Cornwall and Devonshire,* • ;o 

 By John Taylor, Esq. Treasurer of the Geological Society. > 



As I am not aware that the treatment of tin ores, or the mode 

 of smelting them, has been recently described, and as the prac- 

 tice is confined to a certain district, it may be acceptable to the 

 Society to have some account of the processes now used in 

 Cornwall and Devon. 



Tin ores are found in two kinds of deposits ; first in veins 

 accompanied by various other minerals ; and, secondly, in allu- 

 vial matter in detached fragments. 



It is usual in Cornwall not to apply the word ore to the oxide 

 of tin, but to distinguish it, when in that state, by the term 

 Black Tin, in contradistinction to white tin, which appellation is 

 applied to it when smelted and in the metallic state. 



The two kinds of tin ore above mentioned are, therefore, gene- 

 rally known by the names of Mine Tin and Stream Tin ; and as 

 they are for the most part smelted separately, and by different 

 means, and as the metal produced from them is different as to its 

 purity, it may be essential to point out the causes from whiclx. 

 this diversity seems to arise. 



* From the Transactions of the Geological Society. 



New Series, vol. iii. 2 g 



